There are numerous situations in which it is necessary or desirable to employ friction modifiers in lubricant compositions in order to beneficially control frictional characteristics between the two sliding surfaces that are frictionally engageable with each other. For example, the useful life of automatic transmissions can be improved by selection and use of lubricants containing suitable friction modifier systems. However, despite improvements made in the art of friction modification, a need exists for improved friction modifier systems that have the capability of establishing and maintaining a substantially constant frictional characteristics between a pair of friction surfaces that are periodically frictionally engaged with each other such as occurs in the operation of automatic transmission shifting clutches, and like power transmission apparatus. In particular, a need exists for friction modifier systems which have the capability of establishing and maintaining a substantially constant static breakaway coefficient of friction (.mu..sub.s) of such friction surfaces. Moreover another need is for friction modifier systems which have the additional capability of also maintaining a substantially constant ratio between (i) the low speed dynamic (.mu..sub.0) coefficient of friction of such friction surfaces, and (ii) the (midpoint) dynamic coefficient of friction (.mu. .sub.d) of such friction surfaces.
The static breakaway coefficient of friction reflects the relative tendency of engaged parts, such as clutch packs, bands and drums, to slip under load. If this value is too low, the slippage can impair the driveability and safety of a vehicle in which such apparatus is utilized. Likewise, for maintaining proper shift-feel durability, the ratio of the low speed dynamic coefficient of friction (or the coefficient of friction at the end of engagement of friction surfaces) to the (midpoint) dynamic coefficient of friction between the engaged parts should be kept substantially constant during long periods of service in vehicles equipped with such apparatus. The ratio is often called as "static to dynamic ratio" or "rooster tail" in lubrication industry.
The development of effective friction modifiers is an empirical art where few if any guidelines exist, and where predictions concerning the operability of new untested systems are unreliable. Therefore, only after a proposed new system has been tested and found to be effective for its intended usage can valid predictions be made as to the effect of reasonable variations in the makeup of that system.